Due to the deepening of oil wells and gas wells (hereunder, oil wells and gas wells are collectively referred to as “oil wells”), there is a demand to enhance the strength of oil-well steel pipes. Specifically, 80 ksi grade (yield stress is 80 to 95 ksi, that is, 551 to 654 MPa) and 95 ksi grade (yield stress is 95 to 110 ksi, that is, 654 to 758 MPa) oil-well steel pipes are being widely utilized, and recently requests are starting to be made for 110 ksi grade (yield stress is 110 to 125 ksi, that is, 758 to 862 MPa) and 125 ksi grade (yield strength is 862 MPa or more) oil-well steel pipes.
Many deep wells are in a sour environment containing hydrogen sulfide that is corrosive. Oil-well steel pipes that are used in such sour environments are required to have not only a high strength, but to also have sulfide stress cracking resistance (hereunder, referred to as “SSC resistance”).
Steels with enhanced hydrogen embrittlement resistance characteristics (SSC resistance and delayed fracture resistance) are proposed in Japanese Patent Application Publication No. 56-5949 (Patent Literature 1) and Japanese Patent Application Publication No. 57-35622 (Patent Literature 2). The steels disclosed in the aforementioned Patent Literatures contain Co, and thus enhance the hydrogen embrittlement resistance characteristics (SSC resistance and delayed fracture resistance).
Specifically, a high tensile strength steel disclosed in Patent Literature 1 is obtained by quenching and tempering steel having a chemical composition containing C: 0.05 to 0.50%, Si: 0.10 to 0.28%, Mn: 0.10 to 2.0%, Co: 0.05 to 1.50% and Al: 0.01 to 0.10%, with the balance being Fe and unavoidable impurities, and has a yield stress of 60 kg/mm2 or more.
A high-strength oil-well steel disclosed in Patent Literature 2 is obtained by subjecting a steel having a chemical composition containing C: 0.27 to 0.50%, Si: 0.08 to 0.30%, Mn: 0.90 to 1.30%, Cr: 0.5 to 0.9%, Ni: 0.03% or less, V: 0.04 to 0.11%, Nb: 0.01 to 0.10%, Mo: 0.60 to 0.80%, Al: 0.1% or less and Co: 3% or less, with the balance being Fe and unavoidable impurities, in which the impurities contain P: 0.005% or less and S: 0.003% or less, to quenching at 880 to 980° C., and then tempering at 650 to 700° C.
However, in a case where Co is contained in steel with a low C content such as the steels disclosed in Patent Literature 1 and Patent Literature 2, the strength may be insufficient in some cases. Therefore, with respect to oil-well steel pipes for practical use, stable production of 125 ksi grade (yield strength is 860 MPa or more) oil country tubular goods having SSC resistance that can withstand the standard conditions (H2S environment at a pressure of 1 atm) of a constant load test as defined in NACE TM0177 method A has not yet been achieved.
Under the above-described background, in order to obtain a high strength, attempts are being made to use a high-carbon, low-alloy steel that contains an amount of C that is more than 0.45% that has heretofore not been adopted for practical use, for oil country tubular goods.
An oil-well steel pipe disclosed in Japanese Patent Application Publication No. 2006-265657 (Patent Literature 3) has a chemical composition containing, in mass %, C: 0.30 to 0.60%, Si: 0.05 to 0.5%, Mn: 0.05 to 1.0%, Al: 0.005 to 0.10%, Cr+Mo: 1.5 to 3.0% of which Mo is 0.5% or more, V: 0.05 to 0.3%, with the balance being Fe and impurities, the impurities containing 0.025% or less of P, 0.01% or less of S, 0.0010% or less of B and 0.01% or less of O (oxygen), that is produced by subjecting a low-alloy steel having a bainite single-phase metal microstructure to oil-cooled quenching or austempering and thereafter performing tempering. In Patent Literature 3, it is described that, by employing the above-described production method, quench cracking that is liable to occur during quenching of a high-carbon, low-alloy steel can be suppressed, and oil-well steel or an oil-well steel pipe having excellent SSC resistance is obtained.
A steel for oil country tubular goods disclosed in International Application Publication No. WO 2013/191131 (Patent Literature 4) has a chemical composition containing, in mass %, C: more than 0.35% to 1.00%, Si: 0.05% to 0.5%, Mn: 0.05% to 1.0%, Al: 0.005% to 0.10%, Mo: more than 1.0% to 10%, P: 0.025% or less, S: 0.010% or less, O: 0.01% or less, N: 0.03% or less, Cr: 0% to 2.0%, V: 0% to 0.30%, Nb: 0% to 0.1%, Ti: 0% to 0.1%, Zr: 0% to 0.1%, Ca: 0% to 0.01%, and B: 0% to 0.003%, with the balance being Fe and impurities, in which the product of the C content and the Mo content is 0.6 or more. In the aforementioned steel for oil country tubular goods, the number of M2C carbides with a circle-equivalent diameter of 1 nm or more and which have a hexagonal structure is five or more per 1 μm2, and a half-value width of a (211) crystal plane and a C concentration satisfy a specific relation. The aforementioned steel for oil country tubular goods also has a yield strength of 758 MPa or more.
However, there is a demand for higher strength and more excellent SSC resistance than in the steel pipes disclosed in Patent Literatures 3 and 4.
In addition, the conventional evaluation of the SSC resistance of a steel material has been based on, for example, a tensile test or a bending test such as the Method A test or Method B test defined in NACE (National Association of Corrosion Engineers) TM0177. Since these tests use an unnotched test specimen, consideration is not given to SSC propagation arresting characteristics. Therefore, even in the case of a steel material that is evaluated as having excellent SSC resistance in the aforementioned tests, in some cases SSC arises due to propagation of latent cracks in the steel.
Accompanying the deepening of oil wells and the like in recent years, steel material for oil country tubular goods is required to have more excellent SSC resistance in comparison to the steel material used in the past. Consequently, in order to further enhance SSC resistance it is preferable to not only prevent the occurrence of SSC, but to also suppress the propagation of SSC. It is necessary to improve the toughness of steel to suppress the propagation of SSC in the steel. From this viewpoint, a DCB (Double Cantilever Beam) test according to Method D defined in NACE TM0177 is conducted on steel. In the DCB test, steel material for oil country tubular goods to be used under a highly corrosive environment is required to have high fracture toughness (hereunder, referred to as “K1SSC”).